The Mediator is a series of novels for young adults written by Meg Cabot.
The series concerns a girl, Susannah "Suze" Simon. Susannah is capable of speaking to and seeing ghosts that no one else can see, and she helps them complete their "unfinished business" so they can move onto wherever they should be going.
Susannah moves from New York City to California as a result of her mother marrying a man with three sons who lives and works in California. In her new house, she meets and falls in love with a ghost named Jesse who haunts the house because it was the location of his death.
There are six books in the Mediator series.